1. Field of the Invention
The present invention relates to a filter circuit such as a continuous time analog integrated filter circuit, more particularly relates to a Gm-C (Transconductance-C) filter circuit having an integrator as a component, and to a method for realizing a system able to change the cutoff frequency, pole, or zero filter frequency characteristic up to over one hundredfold with a fine resolution of about 1% of a signal bandwidth.
2. Description of the Related Art
In the past, as analog integrated filters mainly using active elements, discrete time filters such as switched capacitor filters (SCFs) and continuous time filters such as transconductance-capacitor (Gm-C: Transconductance-C) filters have been widely applied.
A discrete time analog filter basically performs sampling operations using a clock. Its frequency characteristic is determined by a reference clock frequency having extremely small variation and fluctuation and further a ratio of capacitor values having good matching, so there is the advantage that a filter frequency characteristic having high precision is obtained. On the other hand, there are the defects that a prepositioned filter is required for preventing aliasing due to the sampling operation and that a broadband operational amplifier is necessary in order to realize circuit settling in a clock cycle, so, particularly in a high frequency filter, the power consumption tends to increase.
As opposed to this, a continuous time filter basically performs open loop operations like a Gm-C filter, is able to be configured without using an operational amplifier, and is suitable for higher speeds. It is also free from the problem of aliasing. For this reason, particularly for the purpose of a high speed filter, it can be said that a Gm-C filter or MOSFET-C filter or any other of the series of continuous time filters derived from them is suitable.
For example, in a system for reproduction of a signal from an optical disk such as a CD (compact disk) or DVD (digital versatile disk), an equalization filter is used for pre-processing for converting a reproduction signal to a digital signal. The above continuous time filter is used for this. The main purpose of this filter is to correct information of a bit pattern distorted in the optical-electric conversion path by using the frequency-gain characteristic of the filter and thereby reproduce a bit pattern sequence written on the disk with a low error rate.
A reproduction signal from an optical disk has a certain signal frequency bandwidth corresponding to a physical length of pits formed in the disk. On the other hand, the reproduction frequency per se changes in accordance with the rotational speed or linear speed of the disk at the time of reproduction. For example, when reproducing a signal by setting the disk rotational speed to two times normal, the reproduction signal at the same disk track doubles in frequency. In a CD, DVD, or other optical disk, for shortening the transfer time at the time of copying or buffering data between storage media, the general practice is to reproduce the signal at a speed of tens of times the reference reproduction frequency (in other words, disk rotational speed). At this time, it is necessary to also change the frequency characteristic of the equalization filter proportional to the disk rotational speed.
Specifically, the cutoff frequency of the equalization filter at the time of the reference reproduction of a CD (normal speed of CD) is about 0.7 MHz, the cutoff frequency at the time of the reference reproduction of a DVD (normal speed of DVD) becomes about 4 MHz, and the cutoff frequency at the time of 16× speed of a DVD becomes about 70 MHz. Accordingly, when desiring to realize a signal processing system able to reproduce a signal from a disk from the normal speed of a CD to 16× speed of a DVD by a common equalization filter, it is necessary for the cutoff frequency of the filter to be variable up to one hundredfold.